This invention relates to a flow cell for light absorption measurement, and more particularly, to a flow cell coated with a polymer having an index of refraction below that of water. This flow cell has special application in the well established techniques of high performance liquid chromatography (HPLC) and capillary zone electrophoresis (CZE).
Light absorption detectors for HPLC and CZE generally comprise four basic components: a light source, a means for selecting a narrow increment of wavelengths, a flow cell, typically in the form of a hollow tube, through which a sample to be analyzed and the light are passed, and a light sensor which measures the amount of light transmitted through the flow cell. When a light absorbing component passes through the flow cell,the amount of light transmitted through the flow cell decreases in accordance with Beer's law; ##EQU1## where I is the transmitted light power, I.sub.o is the light power incident on the flow cell, .alpha. is the molar absorptivity of the sample, b is the pathlength of the light in the flow cell (in centimeters), and c is the sample concentration (in moles per liter). The detector output is usually in terms of Absorbance (A) which is defined as the product .alpha. b c and is proportional to both the sample concentration, c, and the pathlength, b. The longer the pathlength, the larger the detector output signal for a given sample concentration. In conventional flow cells, light that strikes the lateral wall of the flow cell is partially lost due to absorption and scattering at the wall. This lost light causes an increase in noise in the output signal of the detector. The lateral dimension or diameter of the flow cell can be increased to reduce the fraction of light striking the lateral wall, but this increases the volume of the flow cell in the proportion to the diameter squared. A larger cell volume results in spreading out or dispersion of a sample peak and loss in chromatographic resolution in HPLC and a similar loss in resolution in CZE. In practice, this loss in resolution limits conventional flow cells to pathlengths of the order of 6 to 10 mm for HPLC and even shorter for CZE because of the narrower sample peaks or smaller peak volumes associated with CZE.
Accordingly, it has long been desired to produce flow cells capable of longer pathlengths without an excessive increase in light loss or cell volume. This desire may be realized by coating the inside wall of the flow cell with a low refractive index polymer so that light striking the coated wall is total internally reflected back in to the cell bore, and light-piped along the cell bore. The basic requirement for light-piping is that the refractive index of the internal wall coating be less than that of the liquid in the flow cell. Water has the lowest refractive index in the UV range of the spectrum for wavelengths between 190 nm and 300 nm of liquids commonly used in HPLC and CZE, so the refractive index of the coating should be less than that of water. A further requirement of the coating is that it is reasonably transparent at the wavelengths used in the measurement of light absorption in the flow cell. While light does not propagate in the coating when total internally reflected, an evanescent wave is established along the surface that will absorb light power if the coating material is not transparent.
Light-piping in a liquid is not a new concept. Commercial liquid light pipes are available, but these usually contain a high refractive index liquid so that polymer coating of TEFLON TFE and TEFLON FEP both of which are available from DuPont Polymers of Wilmington, Del., will effectively light pipe. However, these long available polymers will not effectively light pipe in low refractive index liquids like water as their indices of refraction are greater than that of water.
U.S. Pat. No. 4,575,424 discloses flow cells for light absorption detectors which are internally coated with rhodium which is polished to a mirror finish. However, the flow cell coating is a rather expensive material which requires complicated manufacturing techniques to give a highly polished surface on the inside of a narrow bore. Further, rhodium is not a perfect electrical conductor, and, consequently, there is some absorption of light upon reflection.
Accordingly, the need remains for a flow cell useful for low refractive index liquids which is inexpensively and easily coated with a material having an index of refraction lower than that of water which totally internally reflects light along the cell bore.